1. Field of the Invention
This invention relates to magnetic printers and printheads for use in magnetic printers.
In a typical magnetic printing machine, electrical signals are applied to magnetic recording heads which induce magnetic field variations in the surface of a moving magnetic recording medium. The field variations produce a latent magnetic image on the surface of the recording medium. The magnetic image will attract and retain magnetic ink particles. An ink toner, which is usually supplied in dry particulate form, is applied to the magnetic image and subsequently transferred to paper or other hard copy media. After transfer of the toner from the recording medium to the paper, the recording medium is cleaned of residual toner and erased to prepare it for recording of subsequent magnetic images.
Magnetic printing has several advantages over the more common electrostatic, or xerographic, printing technique. In electrostatic printing, toner particles are attracted to electric fields created by a latent charge image on a dielectric medium. The use of electrostatic fields requires the utilization of high voltages, which creates many potential problems. In addition, many electrostatic printers employ lasers with precision focusing systems, precision scanning systems, servo motor drum control, servo motor paper feed systems and elaborate light shielding, all of which increase the cost of such printers. Magnetic printing, on the other hand, utilizes magnetic rather than electrostatic fields and thus does not require high operating voltages. Furthermore, magnetic printers are mechanically much simpler than electrostatic printers (e.g., simple friction drive is used to synchronize paper and printing drum motion). Potentially, therefore, magnetic printing can be significantly simpler and less expensive than electrostatic printing.
2. Description of the Prior Art
One of the major difficulties which has heretofore prevented the use of magnetic printers is the design of the recording head which records an image onto the recording medium. Unlike recording heads which are used in magnetic audio and video tape recorders and the like, a recording head for a magnetic printer must be extremely wide and include a plurality of print positions across the width of the head in order to record the required image onto the recording medium as it moves past the head. Whereas the maximum tape width used in magnetic tape recorders is generally 1 inch, a magnetic printer is required to print an image having a width on the order of 81/2 inches (standard paper width) or greater. Using typical magnetic recording heads, the current demands for such printing requirements become excessive. In addition, wear problems become critical, and the cost of such heads is extremely high.
One type of head which is designed specifically for use with magnetic printers is disclosed in U.S. Pat. No. 4,138,702 to Magnenet, issued Feb. 6, 1979. In contrast to the normal recording head, which utilizes leakage of magnetic flux from a small gap in a magnetic circuit, the head arrangement disclosed by this patent incorporates an electromagnet having a core which has a recording pole and a flux closing pole, both of which are perpendicular to the surface of the recording medium. The electromagnet forms a closed magnetic circuit with the recording medium in which the magnetic flux is substantially perpendicular to the surface of the recording medium. The flux passes from the recording pole through the recording medium and back to the flux closing pole. The cross-section of the recording pole is much smaller than that of the flux closing pole, thereby causing its flux density to be much greater. The head is designed so that the flux density at the recording pole is sufficient to form a magnetic image on the recording medium, but the flux density on the flux closing pole is insufficient to record an image onto the recording medium.
Although the above design overcomes many of the problems encountered with typical magnetic recording heads when used in magnetic printing, several problems remain. Chief among these is the complexity of connections which result when a plurality of printheads are used. The physical construction of the Magnenet printhead is such that it is difficult to achieve the high density which is required in magnetic printing. Since the printer operates as a dot printer, whereby a plurality of very small closely spaced dots are printed on the recording medium in a predetermined arrangement as the medium moves past the head in order to form the magnetic image, an individual recording head is required for the formation of each dot. Thus, a printhead for a magnetic printer actually incorporates a plurality of individual recording heads, each of which prints at a particular position along a line which is transverse to the direction of travel of the recording medium. For high quality printing, a density on the order of 240 dots per inch is required. The complexity of electrical connections needed to meet such requirements can become excessive. It is a primary object of the present invention to provide a printhead which has a physical configuration that achieves the high dot density required for high quality magnetic printing while reducing the number of electrical connections. A further object of the present invention is to provide a design which can be fabricated by using unsophisticated machine tools, inexpensive components and can be mass produced inexpensively.
After the toner has been applied to the recording medium and subsequently transferred to the paper, any residual toner must be removed from the recording medium and the recording medium must be erased to prepare it for the recording of a subsequent image thereon. Typically, these two functions have been performed separately. The most common way of removing residual toner has been to use a scraper which physically contacts the recording medium and scrapes the residual toner away from the recording medium. The physical contact between the scraper and the recording medium reduces the life of both of these components. In addition, the physical contact causes the scraper and recording medium to form wear-matched surfaces, which requires that both of them be replaced simultaneously.
Some scrapers have been developed which do not physically contact the surface of the recording medium. One such scraper is disclosed in U.S. Pat. No. 3,945,343, issued to Berkowitz on Mar. 23, 1976. In this system, the scraper comprises a rotatable cylinder disposed around a multiple magnetic stator. The stator attracts residual toner to the rotating cylinder. As the cylinder rotates, the toner which located on it is removed by means of a blade which is biased against the surface of the cylinder. The toner is then collected in a container. This system, although it effectively removes residual toner, does nothing to erase the recording medium so as to prepare it for subsequent recording of an image. A separate recording head is required to erase the recording medium.
It is another object of the present invention to provide a scraper which does not contact the recording medium and both removes residual toner and erases the recording medium.